Plastic material is in widespread use in the manufacture of a variety of articles. For example, plastic material is supplied to molding presses which cause the plastic material to be melted and to be molded to various configurations to provide, for example, housings, toys, containers and the like.
Generally, plastic material for use in a molding press is supplied in the form of pellets to a blender. Depending on the mixture sought to be obtained in the blender, additional plastic material in the form of pellets may be supplied. After the pellets of the plastic materials have been blended together to provide a homogeneous mixture, the mixture is conveyed in batch form to a molding press where it is dried and used in the molding of desired products.
In a similar manner, plastic material is supplied to extruders. Extruders may be used, for example, to apply a plastic covering to a metallic or optical fiber transmission medium or to apply one or more jackets to one or a plurality of the transmission media. Generally, the plastic material to be applied by the extruder is in pellet form. The pellets of the plastic material are supplied from a bulk carrier such as a railroad car which may be positioned outside a building in which the extruder is disposed. The pellets may be conveyed by a pneumatic system from the carrier through the portions of the building to the extruder. In the alternative, the pellets may be provided in relatively large cartons which are positioned adjacent to using apparatus in a manufacturing facility. The pellets are deposited in a hopper of the extruder after which they are fed through each of a plurality of heating zones of the extruders to provide a molten mass of the plastic material of which the pellets are comprised. See, for example, U.S. Pat. No. 3,078,514 which issued on Feb. 26, 1963 in the name of A. N. Gray. Then the molten plastic material is moved into a crosshead of the extruder and through a die onto an elongated material which is being moved through the crosshead.
Problems in the feeding of plastic materials have been observed over the years. For example, when it is desired to obtain a particular color molding material for feeding to a molding press, pellets of one color may be blended with pellets of another color. The mixture of the pellets is fed into the hopper of a molding press. Ones of the pellets may become lodged in portions of the hopper or feed arrangement. As a result, when there is a cutover to another color, the pellets which were not removed during a changeover process may become dislodged and contaminate the new color material.
The same kind of problem also may occur when the compositions of materials used in different products may vary. Lodged particles of one composition which become dislodged may contaminate the pellets of another composition and thereby affect adversely the properties of the material provided by the molding press or by the extruder.
In the conventional plastic material supply systems, care also must be taken to remove dust caused by the fines associated with the pellets.
Furthermore, there has been a concern about the cost of a material handling system for plastic material supplied to molding presses. The additional handling required to process material in a blender prior to the feeding of the material to a using apparatus adds to the cost of the final product. The use of pellets is expensive inasmuch as apparatus which is used to pelletize plastic material generally involves relatively high maintenance costs. Also, it should be apparent that the use of pneumatic conveying systems to transport granular material from railroad hopper cars to using apparatus within a building involves substantial capital and maintenance costs.
What is needed and what is not available in the art are methods and apparatus for feeding plastic material to using apparatus in a manner which overcomes problems in present feeding systems. Also, the sought-after methods and apparatus should be such that they do not entail substantial capital expenditures nor involve high maintenance costs.